The present invention relates to hydrostatic power steering systems of the type used to control the flow of fluid from a source of pressurized fluid to a vehicle steering device, such as a cylinder or rotary motor.
A typical hydrostatic power steering system includes a fluid controller of the type having a housing, which defines various fluid ports, and further includes a fluid meter and valving, operable in response to an input, such as the rotation of the vehicle steering wheel. The typical fluid controller also includes an arrangement for imparting follow-up movement to the valving in response to the flow of fluid through the controller valving and the fluid meter to the steering device. The flow of fluid through the controller valving is proportional to the rate at which the steering wheel is rotated.
Steering systems and fluid controllers of the type to which the present invention relates are illustrated and described further in U.S. Pat. No. 3,801,239, assigned to the assignee of the present invention and incorporated herein by reference. The fluid controller of the cited patent was developed in response to a problem which has been referred to as "travel limit slip". When the steering cylinder attached to the vehicle steered wheels reaches the end of its stroke, or hits a mechanical stop, the steered wheels have reached the end of their travel. However, because of leakage out of the fluid meter and valving of the controller, continued application of turning force to the steering wheel by the vehicle operator will result in the steering wheel continuing to turn, although typically at a slower rate. The rate of steering wheel rotation in this condition (against the "stops") is known as the "travel limit slip rate" (TLSR) and is measured in revolutions per minute of the steering wheel.
Those skilled in the art will understand that the above explanation also applies to power steering systems in which the fluid pressure operated device is not a linear steering cylinder, but instead, is a rotary fluid motor, and the invention will be described in connection therewith.
An attempt to overcome the travel limit slip problem, or at least reduce the TLSR, was illustrated and described in U.S. Pat. No. 5,136,844, also assigned to the assignee of the present invention and incorporated herein by reference. The fluid controller of the cited patent is of the type referred to as having a "plugged star", i.e., the gerotor (meter) star receives within a counterbore at its rearward face a plug member. Fluid pressure is communicated to the region adjacent the plug, whenever the controller valving approaches its maximum displacement position, thus biasing the opposite end of the gerotor star into frictional engagement with an adjacent housing surface. Although the fluid controller illustrated and described in U.S. Pat. No. 5,136,844 has been generally successful in reducing the TLSR, the invention of the cited patent does involve a substantial redesign of the fluid controller. For example, the valving must be modified, and additional fluid passages provided in both the housing and the endcap.
Certain hydrostatic power steering systems now available are of the type referred to as "knob control" systems, in which there is typically a steered wheel position sensor, and a steering wheel position sensor. Also, such systems generally include valving to port fluid to or from either the high pressure or low pressure side of the system, as a way of compensating for leakage, etc. As a result, the steered wheels are kept proportionately aligned with (in "registry" with) the "knob" on the steering wheel, as is generally considered desirable by vehicle operators, thus the term "knob control".
On a knob control steering system, the valving which ports fluid typically comprises a pair of electrically controlled auxiliary valves, with the electrical control signal to the valves being in response to the generation of an error signal, wherein the error signal is calculated to reflect any deviation of the steered wheels from that commanded by the position of the steering wheel.